MEMS (Micro-Electro-Mechanical Systems) devices are known having a mirror structure manufactured using the semiconductor material technology.
These MEMS devices are, for example, used in portable apparatuses, such as portable computers, laptops, notebooks (including ultra-thin notebooks), PDAs, tablets, cellphones, and smartphones, for optical applications, in particular for directing light beams generated by a light source with desired modalities.
By virtue of their small dimensions, these devices are able to meet stringent requirements as regards the occupation of space, in terms of area and thickness.
For example, micro-electro-mechanical mirror devices are used in miniaturized projector modules (so called picoprojectors) which are able to project images at a distance or to generate desired patterns of light.
Micro-electro-mechanical mirror devices generally include a mirror element suspended over a cavity and manufactured from a semiconductor material body so as to be mobile, typically with an inclination/tilting or rotation movement, for directing the incident light beam in a desired way.
For example, FIG. 1 schematically shows a picoprojector 9 comprising a light source 1, typically a laser source, generating a light beam 2 made up of three monochromatic beams, one for each base color, which, through an optical system 3 shown only schematically, is deflected by a mirror element 5 in the direction of a display 6. In the example shown, the mirror element 5 is of a two-dimensional type, driven so as to turn about a vertical axis V and a horizontal axis H. Rotation of the mirror element 5 about the vertical axis V generates a fast horizontal scan, as shown in FIG. 2. Rotation of the mirror element 5 about the horizontal axis B, perpendicular to the vertical axis V, generates a slow vertical scan, typically of a saw tooth type.
In a different embodiment of the system of FIG. 1, the system comprises two micromirrors, arranged in sequence on the path of the light beam 2, each of which is able to rotate about an own axis; namely, one is rotatable about the horizontal axis B, and the other is rotatable about the vertical axis V so as to generate the same scanning scheme as shown in FIG. 2.
Another application of micromirror systems is in a three-dimensional (3D) gesture recognition system. These systems normally use a picoprojector and an image acquisition device, such as a camera. The light ray here may be in the range of visible light, invisible light, or any useful frequency. The picoprojector for this application may be similar to the picoprojector 9 of FIG. 1, and the light beam 2 deflected by the micromirror 5 is used for scanning an object in two directions. For example, the picoprojector may project small stripes on the object. Any projecting or recessed areas of the object (due to the depth thereof) create deformations in the light rays detected by the camera, which may be processed by suitable electronics for detecting the third dimension.
In both cases, with the considered technology, rotation of the mirror element is driven via an actuation system, generally of an electrostatic, magnetic, or piezoelectric type.
FIG. 3 shows a mirror element 5 of a biaxial type with generic actuation. Here, a die 10 comprises a suspended region 11 extending over a substrate (not visible) and carrying a reflecting surface 14. The suspended region 11 is supported by a suspended frame 13 by a first pair of arms 12 having elastically deformable portions, which form first torsion springs. The first arms 12 extend on opposite sides of the suspended region 11 and define the rotation axis V of the mirror element 5. The suspended frame 13 is connected to a fixed peripheral portion 15 of the die 10 via a second pair of arms 16 having elastically deformable portions, which form second torsion springs and enable rotation of the suspended frame 13 and of the suspended region 11 about the horizontal axis B. A first actuation structure 18A (shown only schematically and of an electrostatic, magnetic, or piezoelectric type) is coupled to the first arms 12 or to the suspended region 11 and is configured to cause a rotation actuation movement of the first arms 12 about the vertical axis V (parallel to an axis X of a Cartesian reference system XYZ), as a function of first electrical driving signals. A second actuation structure 18B (shown only schematically and of an electrostatic, magnetic, or piezoelectric type) is coupled to the second arms 16 or to the suspended frame 13 and is configured to cause a rotation actuation movement of the second arms 16 about the horizontal axis H (parallel to an axis Y of the Cartesian reference system XYZ), as a function of second electrical driving signals.
Rotation of the mirror element 5 about the vertical axis V that causes the horizontal scan occurs with an angle generally of ±12°, and rotation of the mirror element 5 about the horizontal axis H that causes the vertical scan generally occurs with an angle of ±8°.
To ensure proper operation, the angular position of the mirror element 5 is generally controlled via sensing elements. In fact, minor deviations of the physical or electrical characteristics of the structures, due to the variability in the production lots, to assembly imprecisions, or to different operating conditions, such as temperature or ageing, may lead to even considerable errors in the direction of the emitted light beam.
To this end, sensors of the angular position of the micromirror element are generally integrated in the die 10. Usually, these sensors are based upon capacitive or piezoelectric principles.
Currently, MEMS micromirrors with an increasingly higher optical resolution are required. Since the optical resolution is related to the size of the reflecting surface, this requirement results in the need for increasingly larger reflecting surfaces, up to 7 mm.
However, this entails a considerable and undesirable increase in the overall dimensions of the device, in so far as added to the space occupied by the reflecting surface are the spaces required by the actuation elements, by the position sensors, and by possible other driving, control, and management elements, integrated in the same die.
There is a need in the art to provide a micro-electro-mechanical device and a corresponding manufacturing process that overcome the drawbacks of the prior art.